This application is for a Physician-Scientist Award for basic research related to the control of cardiac gene expression and cardiac development. The applicant is trained as an adult cardiologist with a special expertise in cardiac electrophysiology and pacing. For the past year he has also worked in the Laboratory of Molecular Cardiology at Children's Hospital on thyroid and retinoic acid receptor regulation of myosin heavy chain expression. The proposed research will seek to better define the role of specific nuclear receptors in cardiac gene regulation and cardiac development. The planned research comprises two basic approaches. One is "traditional' molecular biology, and will seek to better define the mechanisms by which the previously defined receptors for thyroid hormone and retinoic acid interact with the regulatory sequences of the myosin heavy chains. Myosin is a major contractile protein in the heart, and is physiologically regulated by thyroid hormone and other stress stimuli; this research may yield information directly relevant to such alterations in cardiac function.The myosin heavy chain genes also represent a particularly powerful system in which to study the general mechanism of transcriptional control, because different isoforms of the gene, expressed in the same cell type at the same time, are subject to antithetic effects by the same receptor. The regulatory region of these genes appear as well to be capable of regulation by receptors for retinoic acid, acting on the same gene sequences responsible for regulation by thyroid hormone receptor. Thus this system of a limited number of target genes and receptors may encompass much of the complexity necessary for physiological and developmental regulation. The proposed studies will try to define those aspects of the regulated gene sequence and the regulating receptor structure that are necessary for this combinational complexity using functional (transfection) and structural (binding) assays. A second component of the research focuses on the more general role that retinoic acid and its receptor may have in cardiac development. Excess administration of retinoic acid during fetal development can result in endocardial cushion defects similar to those associated with the most common morbid human congenital cardiac defects. The consequences of these defects and their repair include conduction block and arrhythmias in addition to hemodynamic compromise. Local gradients of retinoic acid in the developing limb bud can produce structural malformations, and the approach to analyzing the effects of retinoic acid on cardiac development will parallel those studies which defined these limb bud effects. Embryologic manipulations will create local gradients of retinoic acid across the developing cardiac structures, which will be observed for abnormal development. These two approaches are, in fact, complementary, because the ultimate goal is to understand, at the level of gene regulation, the pathologic mechanisms resulting in cardiac developmental or functional abnormalities.